Referring to FIG. 1, Third Generation Partnership Project (3GPP) standards specify procedures for the inter-working between the Universal Mobile Telecommunication System (UMTS) radio access network (RAN), also referred to as UMTS Terrestrial Radio Access Network (UTRAN), enhanced UTRAN (E-UTRAN) which is also referred to as Long Term Evolution (LTE) or System Architecture Evolution (SAE) networks, and older second generation networks specified by 3GPP, such as the Global System for Mobile communications (GSM) and GSM enhanced data rates for GSM evolution (EDGE) radio access networks (GERAN). The standards consider a variety of scenarios including ones in which a wireless transmit/receive unit (WTRU) executes a cell reselection procedure that also leads to a change in access network.
One example includes a WTRU camped on a E-UTRAN cell (in the packet switched (PS) domain) and then, due to low signal quality or signal strength, for example, the WTRU reselects a GERAN cell by way of E-UTRAN-to-GERAN cell reselection. This change in access network results in a new registration process (a Routing Area (RA) update procedure) between the WTRU and the target core network (CN) and the Core Network (CN) entities. As shown in FIG. 1, these CN entities may include any of the following, a serving general packet radio service (GPRS) support node (SGSN), a mobility management entity (MME), a home subscriber server (HSS), a serving gateway, a packet data network (PDN) gateway, a policy control and charging rules function (PCRF), and other IP based services provided by a network operator. A similar procedure is also required when a WTRU is in the Circuit Switched (CS) Domain. In the CS domain, the WTRU executes a Location Update towards a Mobile services Switching Center (MSC)/Visitor Location Register (2G-MSC/VLR) (not shown).
As part of the RA update procedure (or combined RA/LA Update) the WTRU identifies itself to the target CN entities using old identities that were previously allocated to the WTRU. Which identities are included in the RA update request message depend on the originating access network of the WTRU (that is, the access network that the WTRU is handing over from). The identities may include a routing area identity (RAI), a tracking area identity (TAI), a temporary mobile subscriber identity (TMSI), a packet temporary mobile subscriber identity (P-TMSI), or an S-TMSI. These temporary identifiers have different forms based on which access network is used to generate them. The identities will now be described in greater detail.
In legacy systems (such as UTRAN and GERAN), the RA is identified by a RAI. The RAI is a concatenation of several other identifiers. The other identifiers include a mobile country code (MCC), mobile network code (MNC), location area code (LAC), and routing area code (RAC). It is noted that the combination of the MCC and MNC are also referred to as the public land mobile network (PLMN) identity. The RAI can be expressed as follows:<RAI>=<MCC><MNC><LAC><RAC>  Equation (1).
In E-UTRAN, the RA is replaced by tracking area (TA). The TA is identified in E-UTRAN using a TAI. A TAI is a concatenation of several of the above mentioned identifiers and a tracking area code (TAC). The length of the TAC is 16 bits (two octets). The TAI is expressed as follows:<TAI>=<MCC><MNC><TAC>  Equation (2).
Since it is undesirable for security reasons to use the permanent Subscriber Identity Module (SIM) card identity, the International Mobile Subscriber Identity number (IMSI), a TMSI, P-TMSI, or S-TMSI is assigned to a WTRU upon network attachment. A TMSI is generated for use in the CS domain, such as in a CS domain of GERAN and/or UTRAN. A P-TMSI is generated for use in the PS domain, when the WTRU attaches to the PS domain using UTRAN or GERAN as the access network. A WTRU operating in GERAN PS domain will utilize, in addition to the P-TMSI, a temporary logical link identifier (TLLI), which is derived from the P-TMSI. In LTE/SAE networks, the temporary identifier is known as the S-TMSI. The P-TMSI is 4 octets in length, that is, 32 bits. The TLLI is also 4 octets in length (32 bits). The length of the S-TMSI is 5 octets (40 bits).
This difference in length of the temporary identifiers used in the various access networks creates a problem when a WTRU informs a new target network of temporary identifiers used in a previous access network. Accordingly, mapping rules for using various identities while changing access networks are desired.